1. Field of the Invention
The present invention relates to a thin film field effect transistor and a display using the same. Particularly, it relates to a thin film field effect transistor in which an amorphous oxide semiconductor is used for an active layer, and a display using the same.
2. Description of the Related Art
In recent years, flat panel displays (FPDs) have been put to practical use, due to the progress made in liquid crystal and electroluminescence (EL) technologies, etc. Especially, an organic electroluminescence element (hereinafter referred to as an “organic EL element”) formed using a thin film material which emits light by excitation due to application of electric current can provide light emission of high brightness at a low voltage, and thus is expected to achieve reduction in device thickness, weight, and size, and power saving, etc. in wide ranging applications including mobile phone displays, personal digital assistants (PDA), computer displays, car information displays, TV monitors, and general illumination.
These FPDs are driven by an active matrix circuit including field effect-type thin film transistors each using, as an active layer, an amorphous silicon thin film or a polycrystalline silicon thin film provided on a glass substrate. (In the description below, the field effect-type thin film transistor is sometimes referred to as a “thin film transistor” or “TFT”.)
On the other hand, to make the FPD thinner, lighter, and more resistant to breakage, attempts are being made to use a resin substrate which is light in weight and flexible instead of the glass substrate.
However, fabrication of the transistors using thin films of silicon described above requires a thermal treatment process at a relatively high temperature, and it is difficult to form the transistors directly on a resin substrate which is generally low in heat resistance.
Hence, such TFTs have been actively developed using, as a semiconductor thin film, a film of an amorphous oxide, such as an In—Ga—Zn—O-based amorphous oxide, which can be formed at a low temperature, as disclosed in Japanese Patent Application Laid-Open (JP-A) No. 2006-165529 and IDW/AD'05, pages 845-846 (Dec. 6, 2005).
As the films for a TFT made with an amorphous oxide semiconductor can be formed at room temperature, the TFT can be prepared on a film (flexible substrate). Therefore, amorphous oxide semiconductors have been attracting attention as a material for active layers of film (flexible) TFTs lately. Particularly, Prof. Hosono et al. of the Tokyo Institute of Technology have reported that a TFT formed using a-IGZO has a field effect mobility of about 10 cm2/Vs even on a PEN substrate, which is higher than that of an a—Si TFT on glass. Since then, TFTs formed using an amorphous oxide semiconductor have especially drawn attention, especially as film TFTs. (See for example, NATURE, vol. 432, pages 488-492, Nov. 25, 2004.)
However, in the case of using, as for example, a drive Circuit of a display, a TFT formed using a-IGZO, there are the problems that mobility ranges from 1 cm2/Vs to 10 cm2/Vs, which provides insufficient performance, the OFF current is high, and the ON-OFF ratio is low. Particularly, in order to apply such a TFT to a display incorporating organic EL devices, further increase in mobility and improvement in ON-OFF ratio are required.